1. Field of the Invention
This invention relates to a rollformed tubular vehicle bumper beam fabricated of ultra-high strength steel and having a plan view sweep value of more than 10.
2. Description of Prior Art
It has been desired to fabricate bumper beams for vehicles, particularly cars, which are made of high-strength steel. It has been difficult to form such bumper beams out of high-strength steel because bumper beams of the type currently in use require a curvature. It is necessary to rollform such bumper beams to obtain the desired curvature. However, when high-strength steel is rollformed into desired curvature, the material of the beams has been damaged in the process resulting in unacceptable quality. The high-strength obtainable with such steels is desired in order to provide a bumper beam which can be impacted at speeds up to five miles per hour without damage being suffered.
Steel is generally characterized as iron that contains carbon in an amount up to about 1.7 percent as an essential alloying constitute. Other materials are frequently used as additional alloying constituents in steel to achieve desired properties in the final composition.
Low carbon mild steels are relatively ductile and therefore facilitate manufacturing forming processes, such as stamping. Mild steels are widely used in numerous applications which do not require high strength. It is, however, desired for certain applications where high strength is needed to use higher carbon steels, frequently alloyed with other constitutes. Such steels are characterized as "high-strength steels". Within this category there is a class defined as "ultra-high strength steel". Such steel has a minimum yield strength of 100 ksi (100 thousand pounds per square inch). Chemical compositions for such steels vary from one producer to another. Different compositions and thermo-mechanical processing may produce equivalent results. Formability of high strength steels is more difficult than with low carbon mild steels because of greater springback and reduced ductility.
It has been desired to use ultra-high strength steels in vehicle bumper design as set forth in Society of Automotive Engineers paper entitled "Application of Empirical Relationship Developed for Ultra-High Strength Steels in Bumper Design" by Borchelt, Shapiro and Subbaraman. As pointed out in this paper, ultra-high strength sheet steel is processed by rollforming because of limited ductility. Current vehicle bumper design requires that bumper beams be curved. This curvature is defined as "sweep". It is desired to fabricate bumper beams having relatively high plan view sweep values (thus greater curvature). However, as pointed out in the paper, current production methods limit the maximum sweep of an ultra-high strength steel bumper beam to a sweep value of 10.